Electronically tunable optical filters have been constructed so that an incident light beam of a first polarization is diffracted by an acoustic wave in a birefringent crystal into a second polarization for a selected bandpass of optical frequencies. The center wavelength of the passband of this type of filter is electronically tunable by changing the frequency of the acoustic wave within the crystal. A unique feature of such acousto-optic tunable filters (AOTFs) is that they are capable of selecting multiple passbands simultaneously and independently. As such, the AOTF is most promising for wavelength selective switching and routing in wavelength division multiplexing (WDM) networks.
Two basic types of AOTFs have been constructed: collinear and noncollinear. In the collinear AOTF, the incident and diffracted optical waves are collinear with the acoustic wave. The diffracted light beam at the selected passband is separated from the incident light beam by crossed polarizers. The collinear AOTF is disclosed in an article entitled "Acousto-Optic Tunable Filters," appearing on pages 744-747 in the June, 1969 issue of The Journal of the Optical Society of America (Vol. 59, No. 6), and in U.S. Pat. No. 3,679,288, entitled "Tunable Acousto-Optic Method and Apparatus."
In the noncollinear AOTF, the optical waves are noncollinear with the acoustic wave. The diffracted beam at the passband is selected from the incident light beam either by crossed polarizers or by spatial separation. The noncollinear AOTF is described in an article entitled "Noncollinear Acousto-Optic Filter with Large Angular Aperture," appearing on pages 370-372 of the Oct. 15, 1974 issue of the Applied Physics Letters (Vol. 25), and in U.S. Pat. No. 4,052,121 entitled "Noncollinear Tunable Acousto-Optic Filter." This type of noncollinear AOTF realizes a large angular aperture by choosing the tangents to the loci of the incident and diffracted light wavevectors to be parallel, a condition known as noncritical phase matching (NPM). Inside the crystal, the group velocity of the incident light beam is collinear with the diffracted light beam.
Another type of noncollinear AOTF known as the critical phase matching (CPM) type is described in U.S. Pat. No. 3,953,107. The CPM type noncollinear AOTF has a small angular aperture and must be used for a well-collimated light source. Inside the crystal of the CPM type AOTF, the incident light beam makes an angle with respect to the diffracted light beam.
Further efforts have been directed at improving the performance of noncollinear AOTFs. These include the use of external acoustic prisms as described in U.S. Pat. No. 4,685,772 entitled, "Tunable Acousto-Optic Filter with Improved Spectral Resolution and Increased Aperture," and the use of internal acoustic reflection as described in U.S. Pat. No. 4,720,177 entitled, "Tunable Acousto-Optic Filter Utilizing Internal Mode Conversion."
One drawback of the noncollinear AOTF is the limited interaction length caused by acoustic beam walkoff from the optical beam. A new type of noncollinear AOTF using an acoustically anisotropic medium is disclosed in an article entitled, "Collinear Beam Acousto-Optic Tunable Filter," appearing on pages 1255-1256 of the Jun. 18, 1992 issue of Electronic Letters (Vol. 28, No. 13) and in U.S. Pat. No. 5,329,397. In the collinear beam (CB) AOTF, the acoustic group velocity is chosen to be collinear with optical ray. This configuration extends the interaction length and achieves high spectral resolution and low drive power. The CBAOTF thus combines the advantages of both collinear and noncollinear type of AOTFs.
For use as a key WDM component in fiberoptic communication networks, the prior art AOTF is yet inadequate due to these basic limitations; insufficient resolution, high sidelobes, and in particular, polarization dependence. The operation characteristics of the AOTF are inherently sensitive to the polarization of the incident light since its operation is based on birefringent diffraction in an optical anisotropic medium.
A polarization independent (PI) AOTF can be constructed by using a polarization division configuration (PDC). In this configuration, the polarized input beam is divided by an input polarizing beam splitter (PBS) into two separate beams of orthogonal polarizations, these two beams pass through two AOTFs or a dual channel AOTF in two separate optical paths and are diffracted at the selected wavelengths. The two diffracted beams are then combined by an output PBS and appear as the filtered unpolarized beam. However, the prior art PIAOTF has severe performance limitations that include large polarization dependent loss (PDL), polarization mode dispersion (PMD), high coherent crosstalk and fabrication difficulties. This is explained with reference to FIG. 1, which shows diagramatically, a prior art PIAOTF as described in the aforementioned U.S. Pat. No. 5,329,397.
The PIAOTF is comprised of a PBS 11, a dual channel AOTF made of an optically birefringent crystal 12 and a PBS 13. A pair of acoustical transducers 14 and 15 are mounted in intimate contact with the birefringent crystal 12 and are connected to a suitable radio frequency generator 16, which may be two separately tuned voltage controlled oscillators (VCOs). The transducer launches a pair of first acoustic waves which are reflected from the output optical face 17 into a second pair of acoustic waves with phase velocity V.sub.p and group velocity V.sub.g. The orientation of the optical face 17 is properly chosen so that the group velocity of the second acoustic wave V.sub.g is along the center axis of the birefringent crystal 12. An incident unpolarized optical beam 18 is split by the input PBS 11 into two separate beams of orthogonal polarizations, an o-ray 19 and an e-ray 20. The two beams are diffracted at the selected passband wavelength and transmitted out of the crystal as the filtered e-ray 21 and o-ray 22. The two diffracted light beams are recombined by the output PBS 13 into the filtered, unpolarized beam 23.
The prior art PIAOTF described above is deficient since the operation characteristics of the AOTF are asymmetrical for the incident o- and e-rays. This polarization asymmetry is true for all types of noncollinear AOTF. For instance, FIG. 2 of the above-cited U.S. Pat. No. 5,329,397 shows the driving acoustic frequency of the prior art AOTF as a function of the optical incidence angle .theta..sub.i. For the same .theta..sub.i, the frequencies .function..sub.o and .function..sub.e for the o- and e-rays are different. For operation with the same driving frequencies, different incidence angles for the o- and e-rays must be properly chosen. In practice, separate precision angular adjustment means are required. This greatly increases the construction complexity of the PIAOTF. Furthermore, due to the difference in angular and frequency characteristics for the o-ray and e-ray, the intensities of the filtered e-ray 21 and o-ray 22 in the two optical paths of the polarization division configuration are in general not equal. Thus, the prior art PIAOTF has a large polarization dependent loss (PDL).
Another deficiency of the prior art PIAOTF shown in FIG. 1 is the amplitude modulation due to coherent optical interference. A basic characteristic of AO interaction is that the optical frequency of the diffracted light is Doppler-shifted by the acoustic frequency .function..sub.a. The signs of the frequency shift for two co-propagating o- and e-rays are different, i.e. one upshift and one downshift in frequency. Referring to FIG. 1, when the two diffracted optical beams 21 and 22 are recombined by the output PBS 13, due to the finite leakage, the two diffracted beams with opposite Doppler shifts will interfere and result in large amplitude modulation at twice the acoustic frequency. For fiberoptic communication network applications, this coherent optical interference must be suppressed.
A further drawback of the prior art PIAOTF due to polarization asymmetry is the polarization mode dispersion (PMD) i.e., the polarization dependent group delay. It is shown in FIG. 1 that the prior art PIAOTF uses a co-planar PDC wherein the acoustic and optical beams for the two polarizations are in the same plane of AO interaction. Furthermore, the optical face 17 is slanted so that the reflected acoustic beam is collinear with the optic axis of the crystal. This results in different optical paths for the o- and e-rays. This difference in group delay for the two polarizations gives rise to a finite PMD, which will severely degrade the performance of the AOTF when used for wideband fiber optic communication applications.
The use of the slanted optical face 17 also causes a large deflection of the transmitted and diffracted beams. To correct the prism effect, an In-Line configuration is desired wherein the transmitted or diffracted light beam is aligned with the incident beam. The In-Line configuration provides the advantage of easy optical alignment and reduces the angular dispersion. Furthermore, the prior art AOTF uses a collinear beam configuration. Since the CBAOTF is comprised of an elongated crystal block, it can provide an extended optical path and thus realizes the high resolution required for WDM applications. However, it is also desirable to extend the In-Line elongated block configuration to the wider class of noncollinear AOTFs for additional advantages such as lower sidelobes.
From the above discussions, it is fair and accurate to state what is needed in the art and is not currently available is an AOTF that meets the requirements for use in WDM applications.